Solution and NMR

Passage of dinitrogen into this mixture precipitates the dinitrogen complex as a blue-black microcrystalline solid, and thus provides a synthesis of pure CpfTi. The yellow-orange CpfTi turns yellow-green in solution, and NMR measurements on the paramagnetic Ti(II) species have led to suggestions of the tautomerism ... 

Olivier L, Poupko R, Zimmermann FI and Luz Z 1996 Bond shift tautomerism of bibullvalenyl in solution and in the solid-state—a C-13 NMR-study J. Phys. Chem. 100 17 995-18 003... 

Sachleben J R ef a/1998 Solution-state NMR studies of the surface structure and dynamics of semiconductor nanocrystals J. Phys. Chem. B 102 10 117... 

The Diels-Alder reaction provides us with a tool to probe its local reaction environment in the form of its endo-exo product ratio. Actually, even a solvent polarity parameter has been based on endo-exo ratios of Diels-Alder reactions of methyl acrylate with cyclopentadiene (see also section 1.2.3). Analogously we have determined the endo-exo ratio of the reaction between 5.1c and 5.2 in surfactant solution and in a mimber of different organic and acpieous media. These ratios are obtained from the H-NMR of the product mixtures, as has been described in Chapter 2. The results are summarised in Table 5.3, and clearly point towards a water-like environment for the Diels-Alder reaction in the presence of micelles, which is in line with literature observations. 

Because six membered rings are normally less strained than five membered ones pyranose forms are usually present m greater amounts than furanose forms at equilib rium and the concentration of the open chain form is quite small The distribution of carbohydrates among their various hemiacetal forms has been examined by using H and NMR spectroscopy In aqueous solution for example d ribose is found to contain the various a and p furanose and pyranose forms m the amounts shown m Figure 25 5 The concentration of the open chain form at equilibrium is too small to measure directly Nevertheless it occupies a central position m that mterconversions of a and p anomers and furanose and pyranose forms take place by way of the open chain form as an inter mediate As will be seen later certain chemical reactions also proceed by way of the open chain form... 

FIGURE 25 5 Distribution of furanose pyranose and open chain forms of d ribose in aqueous solution as mea sured by H and NMR spectroscopy... 

Another technique often used to examine the stmcture of double-heUcal oligonucleotides is two-dimensional nmr spectroscopy (see AfAGNETiC SPIN resonance). This method rehes on measurement of the nuclear Overhauser effects (NOEs) through space to determine the distances between protons (6). The stmcture of an oligonucleotide may be determined theoretically from a set of iaterproton distances. As a result of the complexities of the experiment and data analysis, the quality of the stmctural information obtained is debated. However, nmr spectroscopy does provide information pertaining to the stmcture of DNA ia solution and can serve as a complement to the stmctural information provided by crystallographic analysis. 

Sjitberg and co-workers combined emf and nmr data to develop a model to describe polymerization and protonation of siUcate species in sodium sihcate solutions (43,44). Thirteen polynuclear species are described. This model yields useful insights into the changes in polymerization of sodium siUcate solutions as and [Si02] change. Higher order polynuclear species predominate as [Si02] increases or pH drops under alkaline conditions. The... 

The hydrolysis of the uranyl(VI) ion, UO " 2> has been studied extensively and begins at about pH 3. In solutions containing less than lO " M uranium, the first hydrolysis product is the monomeric U02(OH)", as confirmed using time-resolved laser induced fluorescence spectroscopy. At higher uranium concentrations, it is accepted that polymeric U(VI) species are predominant in solution, and the first hydrolysis product is then the dimer, (U02)2(0H) " 2 (154,170). Further hydrolysis products include the trimeric uranyl hydroxide complexes (U02)3(0H) " 4 and (1102)3(OH)(154). At higher pH, hydrous uranyl hydroxide precipitate is the stable species (171). In studying the sol-gel U02-ceramic fuel process, O nmr was used to observe the formation of a trimeric hydrolysis product, ((U02)3( -l3-0)(p.2-0H)3) which then condenses into polymeric layers of a gel based on the... 

Although the UV spectra of pseudouracil , in comparison with some of its methylated derivatives, suggested that it might be a mixture of the dioxo form (58) and a hydroxyoxo form (64AJC567) and NMR data seemed to uphold this in general terms 66JOC175), it now seems clear on extended criteria that the betaine (59) is probably the main contributor in solution <66JCS(B)565). 

The NMR spectrum of quinoxaline has been measured in CDCI3 and the chemical shift values are as shown in (16) (69JA6381). Curiously, C NMR spectra of phenazine and its derivatives have been recorded in benzene solution and the chemical shift values quoted relative to benzene however, for consistency the values in (17) are quoted relative to TMS. 

A gene encoding this sequence was synthesized and the corresponding protein, called Janus, was expressed, purified, and characterized. The atomic structure of this protein has not been determined at the time of writing but circular dichroic and NMR spectra show very clear differences from B1 and equally clear similarities to Rop. The protein is a dimer in solution like Rop and thermodynamic data indicate that it is a stably folded protein and not a molten globule fold like several other designed proteins. 

Annulene, [22]annulene, and [24]annulene have all been reported. The NMR spectrum of [22]annulene is consistent with regarding the molecule as aromatic, whereas those of the [20] and [24] analogs are not. In each case, there is some uncertainty as to the preferred conformation in solution, and the NMR spectra are temperature-dependent. Although the properties of these molecules have not been studied as completely as those of the smaller systems, they are consistent with the predictions of the Huckel rule. 

A solution of estradiol (38, 15 mg) in methanol-OD (4 ml) and one drop of 10% deuteriosulfuric acid in deuterium oxide is heated under reflux for 5 days. After cooling the reaction mixture is diluted with ether, washed with dilute sodium bicarbonate solution and water, then dried over anhydrous sodium sulfate. Evaporation of the ether gives crystalline 2,4-d2-estradiol (39, 15 mg, 99%), mp 173-175° (ether-hexane), exhibiting 82% isotopic purity and only one aromatic hydrogen by NMR. (For an experimental procedure describing the exchange of aromatic protons under Clemmensen conditions, see section III-D.)... 

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